With increasing population and industrialization over the past two decades, demand and supply for fresh water have increased gradually. Particularly, seawater desalination has been very actively investigated. Membrane distillation has attracted great attention as the most promising process for desalination of highly saline waters.
Membrane distillation is a membrane separation process in which only a particular component (mainly water vapor) in a mixture is able to selectively pass through a porous hydrophobic separation membrane. The driving force in the membrane distillation process for separation and purification of the mixture is the vapor pressure difference between permeable components induced by the temperature difference between both ends of the membrane. This process has various applications, such as wastewater treatment and in the food industry, in addition to seawater desalination.
Such membrane distillation processes are operated at relatively low temperatures in comparison to conventional other distillation processes. Membrane distillation enables the production of fresh water at low cost because it is not a pressure-driven process, unlike reverse osmosis processes. Membrane distillation processes use separation membranes having a smaller pore size than that of separation membranes used in reverse osmosis processes. Accordingly, membrane distillation processes have the advantage that fouling of the membranes can be minimized. However, membrane distillation processes suffer from the disadvantages of relatively low permeate flux and high heat loss during operation due to increased mass transfer resistance, compared to reverse osmosis processes (Non-Patent Document 1).
Thus, separation membranes for membrane distillation are required to have low thermal conductivity and excellent thermal and chemical stability in order to minimize mass transfer resistance and heat loss.
On the other hand, there have been attempts to apply stiff glassy wholly aromatic organic polymers with excellent thermal and chemical properties, such as polybenzoxazole, polybenzimidazole, and polybenzothiazole, to separation membranes (Non-Patent Document 2). However, most of these organic polymers are poorly soluble in general organic solvents. This poor solubility causes difficulties in making membranes by a simple and practical solvent casting method. Only a few membranes using the organic polymers are mostly used for gas separation.
In an effort to overcome such difficulties, a method for fabricating a polybenzoxazole membrane by thermally rearranging a blend membrane of a polyimide having hydroxyl groups in the ortho positions and a poly(styrene sulfonic acid) at 300 to 650° C. (Patent Document 1). However, Patent Document 1 fails to specifically disclose imidization of the hydroxyl polyimide as a precursor for the fabrication of the polybenzoxazole membrane. The applicability of the separation membrane is also limited to gas separation.
The present inventors have also succeeded in fabricating thermally rearranged polybenzoxazole membranes from polyimides having hydroxyl groups in the ortho positions and have also reported that the polybenzoxazole membranes have carbon dioxide permeabilities 10 to 100 times higher than conventional polybenzoxazole membranes fabricated by solvent casting (Non-Patent Document 3).
However, the polybenzoxazole membranes and the thermally rearranged polybenzoxazole membranes described in the prior art documents have limited applicability to gas separation and their applicability to membrane distillation and performance are neither disclosed nor suggested in the above documents and related documents thereof.
In view of this situation, the present inventors have conducted intensive studies to solve the problems of the prior art, and as a result, found that thermally rearranged poly(benzoxazole-co-imide) membranes have excellent thermal and chemical properties and can be fabricated into porous hydrophobic separation membranes for membrane distillation processes in various applications, such as seawater desalination. The present invention has been accomplished based on this finding.
(Patent Document 1) Korean Patent Publication No. 10-2012-0100920
(Non-Patent Document 1) A. Alkhudhiri et al., Desalination 287, 2-18 (2012)
(Non-Patent Document 2) J. P. Critchley, Prog. Polym. Sci. 2, 47-161 (1970)
(Non-Patent Document 3) Y. M. Lee et al., Science 318, 254-258 (2007)